Expandable interbody fusion device with graft chambers

ABSTRACT

An expandable interbody fusion device includes superior and inferior plates that are configured to receive a sequentially inserted stack of expansion members or wafers in interlocking engagement. The superior and inferior plates have openings therethrough in communication with aligned holes through the wafers for receipt and containment of bone graft to promote fusion between opposing vertebral bodies. One of said superior and inferior endplates has a multi-contoured opening extending therethrough.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/689,046, filed Nov. 29, 2012, now U.S. Pat. No. 8,715,351, the entirecontents of which are incorporated by reference herein.

FIELD OF THE INVENTION

The subject invention relates generally to the field of spinal implantsand more particularly to expandable interbody fusion devices with graftchambers.

BACKGROUND OF THE INVENTION

Spinal implants such as interbody fusion devices are used to treatdegenerative disc disease and other damages or defects in the spinaldisc between adjacent vertebrae. The disc may be herniated or sufferingfrom a variety of degenerative conditions, such that the anatomicalfunction of the spinal disc is disrupted. Most prevalent surgicaltreatment for these conditions is to fuse the two vertebrae surroundingthe affected disc. In most cases, the entire disc will be removed,except for a portion of the annulus, by way of a discectomy procedure. Aspinal fusion device is then introduced into the intradiscal space andsuitable bone graft or bone substitute material is placed substantiallyin and/or adjacent the device in order to promote fusion between twoadjacent vertebrae.

Certain spinal devices for achieving fusion are also expandable so as tocorrect disc height between the adjacent vertebrae. Examples ofexpandable interbody fusion devices are described in U.S. Pat. No.6,595,998 entitled “Tissue Distraction Device”, which issued on Jul. 22,2003 (the '998 patent), U.S. Pat. No. 7,931,688 entitled “ExpandableInterbody Fusion Device”, which issued on Apr. 26, 2011 (the '688patent), and U.S. Pat. No. 7,967,867 entitled “Expandable InterbodyFusion Device”, which issued on Jun. 28, 2011 (the '867 patent). The'998 patent, the '688 patent and the '867 patent each disclosessequentially introducing in situ a series of elongate inserts referredto as wafers in a percutaneous approach to incrementally distractopposing vertebral bodies to stabilize the spine and correct spinalheight, the wafers including features that allow adjacent wafers tointerlock in multiple degrees of freedom. The '998 patent, the '688patent and the '867 patent are assigned to the same assignee as thepresent invention, the disclosures of these patents being incorporatedherein by reference in their entirety.

Certain interbody fusion devices also include hollow portions orchambers that are filled with suitable material such as bone graft topromote fusion between vertebral bodies. The extent and size of thechambers establish areas of contact that are configured so as to assuremaximum contact between the bone graft and the vertebral bodies.Sufficient surface area of the device surrounding the chambers needs tobe maintained in order to provide an appropriate load bearing surface towithstand the compressive forces exerted by the opposing vertebralbodies. In addition, where expandable interbody fusion devices are usedto correct height within the intradiscal space, the effect of shearforces on the expanded device due to torsional movement of the spinealso needs to be considered.

Accordingly, there is a need to develop expandable interbody fusiondevices with bone graft chambers that take into account and balancethese factors.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved expandabledevice with openings serving as bone graft chambers for implantationinto the intradiscal space between two opposing vertebral bodies of aspine.

DESCRIPTION OF THE FIGURES

FIGS. 1 and 2 are rear and front perspective views respectively of anexpandable interbody fusion device in unexpanded condition in accordancewith one embodiment of the present invention.

FIG. 3 is a front perspective view of the expandable interbody fusiondevice of FIG. 2 in expanded condition.

FIG. 4 is a cross section of the inferior endplate of the interbodyfusion device as seen along the viewing lines IV-IV of FIG. 3.

FIG. 5 is a top plan view of the interbody fusion device of FIG. 1.

FIG. 6 is a bottom plan view of the interbody fusion device of FIG. 1.

FIG. 7 is a top perspective view of an interlocking wafer serving as anexpansion member to expand the interbody fusion device to the expandedcondition shown in FIG. 3.

FIG. 8 is a bottom perspective view of the interlocking wafer shown inFIG. 7.

FIG. 9 is cross section of the interlocking wafer of FIG. 7 as seenalong viewing lines IX-IX of FIG. 7.

FIG. 10 is a cross section of the expanded interbody fusion device asseen along the viewing lines IV-IV of FIG. 3.

FIG. 11 is a cross section of the expanded interbody fusion device asseen along the viewing lines XI-XI of FIG. 3.

FIGS. 12 and 13 are rear and front perspective views respectively of anexpandable interbody fusion device in unexpanded condition in accordancewith another embodiment of the present invention.

FIG. 14 is a front perspective view of the expandable interbody fusiondevice of FIG. 13 in expanded condition.

FIG. 15 is a top plan view of the interbody fusion device of FIG. 12.

FIG. 16 is a bottom plan view of the interbody fusion device of FIG. 12.

FIG. 17 is a cross section of the expanded interbody fusion device asseen along the viewing lines XVII-XVII of FIG. 14.

FIG. 18 is a cross section of the expanded interbody fusion device asseen along the viewing lines XVIII-XVIII of FIG. 14.

DESCRIPTION OF THE EMBODIMENTS

For the purposes of promoting and understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and described in the following written specification. It isunderstood that no limitation to the scope of the invention is therebyintended. It is further understood that the present invention includesany alterations and modifications to the illustrated embodiments andincludes further applications of the principles of the invention aswould normally occur to one skilled in the art to which this inventionpertains.

In accordance with one embodiment of the invention, an expandableinterbody fusion device 10 includes a first superior endplate 12 and asecond inferior endplate 14, as shown in FIGS. 1-3. The superior outersurface 12 a and the inferior outer surface 14 a of the endplates 12 and14 each define engagement ribs 12 b and 14 b that are configured toengage or grip the vertebral endplates of opposed vertebral bodies in aspine. Preferably, the ribs 12 b and 14 b, as shown also in FIGS. 5-6are configured to prevent expulsion of the device 10 under normal spinalloads. For instance, the ribs as shown may have a pyramidalconfiguration or they may include a saw tooth shape that is inclinedtoward the opening through which the device is inserted into theintradiscal space between opposing vertebral bodies.

The interbody fusion device 10 has a height across the superior andinferior endplates 12, 14 in the unexpanded condition that is less thanthe normal anatomic height of a typical intradiscal space. The inventioncontemplates that a series of expansion members, such as interlockingwafers 100 as will be described, are introduced into the device 10 todistract the opposing vertebrae by separating the superior and inferiorendplates 12, 14 in situ. Insertion of the wafers 100 separates theendplates 12, 14 to expand the height of the device within theintradiscal space and to ultimately restore the normal anatomic heightof the disc space. Expansion devices of this type are shown anddescribed in the '998 patent, the '688 patent and the '867 patentdescribed hereinabove and incorporated herein by reference.

The present invention contemplates an improved interbody fusion device10 that particularly includes openings and holes that define graftchambers for containment of materials that promote bone fusion throughthe device between opposing vertebral bodies. The inferior endplate 14of the interbody fusion device 10 as shown in FIGS. 1-3 is elongate andcomprises a pair of oppositely spaced apart sidewalls 16 and 18extending along the longitudinal direction and projecting upwardly fromouter surface 14 a. A pair of spaced apart end walls 20 and 22 extendlaterally across the device and project upwardly from the outer surface14 a. End wall 20 is disposed at the rear or proximal end of the device10 and end wall 22 is disposed at the front or distal end of the device10. The side walls 16, 18 together with rear end wall 20 and front endwall 22 form an open, upwardly facing full bounded cavity 24 as shown inFIGS. 3 and 4.

The inferior plate 14 as shown in FIGS. 1 and 4 defines a wafer channel26 through the rear end wall 20 and through which the wafers 100 whichserve as expansion members are introduced. The inferior endplate 14includes a pair of opposite ledges 28 that define an upper supportsurface on which each wafer 100 is supported as it introduced into thewafer channel 26, as will be described. The inferior endplate 14 alsodefines an inserter channel 30 that is below and in communication withthe wafer channel 26. The ledges 28 define the bottom surface of thecavity 24. The inserter channel 30 receives a wafer track (not shown)for introduction of the wafers 100 as described in the '688 patentincorporated herein by reference. More specifically the inserter channel30 includes a number of severable posts 32 projecting upward thereinthat are configured to engage an insertion plate and a release plate ina manner as described in the '688 patent.

By continued reference to FIGS. 3-4, it can be seen that each oppositeside wall 16, 18 of the inferior endplate 14 has on an interior surface16 b thereof a pair of ribs 34 projecting into the cavity 24, the ribs34 being spaced lengthwise on the interior of each side wall 16, 18.Each side wall 16, 18 further defines upper surfaces 16 a and 18 aextending lengthwise thereon and generally parallel to each other. Rearend wall 20 defines a recess 20 a extending therein at that the uppersurface thereof and front end wall 22 defines a recess 22 a extendingtherein at the upper surface thereof.

As shown particularly in FIGS. 4 and 6, the inferior endplate 14includes graft chambers defined by a pair of spaced openings 36extending in alignment along the longitudinal direction. The openings 36extend through the outer surface 14 a of the inferior endplate 14,through the support surfaces defined by ledges 28 and communicate withthe cavity 24. A web 38 extends between the pair of openings 36, the web38 supporting one of the posts 32 that is used to engage an insertionplate and a release plate as indicated hereinabove. One other post 32 isdisposed between a first opening 36 and the rear end wall 20 and afurther post 32 is disposed between the second opening 36 and the frontend wall 22, the three posts 32 and two openings 36 extending linearlyalong the longitudinal direction of the inferior endplate 14. Eachopening 36 includes therearound a countersink surface 36 a as shown inFIG. 6 for receipt of bone graft to thereby increase the surface area ofcontact between such bone graft and the endplate of a vertebral body. Atleast one radiopaque marker 39 may be included on the inferior endplate14, such as adjacent the proximal end of side wall 16 and distal end ofsidewall 18, as shown in FIGS. 1-2 to assist in the visualization of theinsertion of device 10 into the intradiscal space.

The superior endplate 12 as shown in FIGS. 1-3 and 10-11 is elongate andcomprises a hub 40 having pair of side surfaces 42 and 44 extendinglongitudinally on each side of the hub 40 and a pair of end surfaces 46and 48 extending respectively at the proximal end and the distal end ofthe superior endplate 12. The hub 40 is sized and configured to fitwithin the cavity 24 in relatively close fit between the side walls 16and 18 and the end walls 20 and 22 of the inferior endplate 14. Thelower surface 50 of the hub 40 (FIG. 11) includes a shaped configurationdefined by wafer mating features 52 that are substantially identical tothe mating features on the lower surface of each wafer 100, as will bedescribed. Superior endplate 12 includes a flange 54 projectingoutwardly and longitudinally from the hub 40 at the rear proximal endsurface 46 and a flange 56 projecting outwardly and longitudinally fromthe hub 40 at the front distal end surface 48. The hub 40 defines agroove 58 and 60 as shown in FIGS. 3 and 11 extending along each side 42and 44 thereof that is configured to engage the ribs 34 of the inferiorendplate 14. This engagement temporarily holds the superior and inferiorendplates together as the device 10 is introduced into the intradiscalspace to be distracted.

As shown particularly in FIGS. 1-3 and 5, the superior endplate 12includes a graft chamber defined by a multi-contoured opening 62extending through the upper surface 12 a and the lower surface 50. Theopening 62 is considered multi-contoured since it has one configurationopening at the upper surface 12 a and a different configuration openingat the lower surface 50. In this particular arrangement, upper portionof opening 62 is a single, oval-shaped opening 62 a. The lower portionof opening 62 comprises a pair of generally circular openings 62 bseparated by a cross member 64 disposed generally centrally andtransversely across superior endplate 12 in the lateral direction.Openings 62 b extend generally linearly along the longitudinal directionof superior endplate 12 and are disposed therethrough such that inassembly with inferior endplate 14 openings 62 b are generally inalignment in the elongate direction with openings 36 through inferiorendplate 14. While the provision of cross member 64 may tend to reducethe area through which the bone graft may flow upon injection into thedevice 10, it should be appreciated that the cross member 64 at thelower portion of superior endplate 12 provides strength to the superiorendplate 12 while allowing greater bone graft area at the outer surface12 a. In addition, cross member 64 allows for a locking location for theuppermost wafer 100, as will be described. It should also be appreciatedthat other multi-contoured shapes of openings may be used asalternatives to the illustrated and described multi-contoured opening62. Radiopaque markers 66 may be included on the outer surface 12 a ofsuperior endplate 12 as shown, for example in FIG. 5.

Details of an interlocking wafer 100 are shown in FIGS. 7-9. The wafer100 is elongate and has an upper surface 102 and a lower surface 104,both of which are generally planar so that the wafers can form a stablestack within the interbody fusion device 10. The trailing proximal end106 includes a downward-facing sloped surface 108 that correspondsangularly to an upward-facing surface 110 on the leading distal end 112of the wafer 100. The two sloped surfaces help displace an earlierinserted wafer 100 upon introduction of a new wafer. More specifically,when a first wafer is within the wafer channel 26, resting on the ledges28 (FIG. 11), the downward-facing sloped surface 108 thereof is liftedupon contact with the upward-facing slope 110 of a newly inserted wafer.This allows the newly inserted wafer to ride along the ledges 28 untilit is positioned fully underneath the previous wafer. The wafer 100further includes notches or indentations 114 that are configured toreceive the ribs 34 on the inner side walls 16, 18 of the inferior plate14 (see FIG. 11) in a manner similar to the grooves 58 and 60 in the hub40 of the superior endplate 12.

The wafer 100 includes several features for interlocking engagement tothe hub 40 and to adjacent wafers 100 in a complementary interlockingmating interface. One particular feature includes a series of lockingelements defined by resiliently deflectable prongs 116 that projectoutwardly above the upper surface 102 of the wafer. In one arrangement,the prongs 116 are disposed along the wafer 100, extending lengthwise inalignment and defining a plurality of resiliently deflectable lockingsurfaces therealong. The lower surface 104 of each wafer 100 as shown inFIGS. 8 and 11 also defines a T-slot configuration for mating with aT-bar configuration on the upper surface 102 of a successive wafer 100.It should be appreciated that the respective T-bar and T-slotconfigurations may be formed on either the upper surface or the lowersurface of a wafer 100 as desired. The structure and function of a wafer100 and the prongs 116 are more fully described in the '867 patent,incorporated herein by reference. In the illustrated arrangement, thereare three prongs 116 extending generally linearly along the elongatelongitudinal direction. A pair of holes 118 extends through the uppersurface 102 and the lower surface 104 of each wafer 100. The holes 118are provided to allow bone graft to flow through the wafers, the holes118 being disposed along the longitudinal direction with at least onehole 118 being situated between each pair of prongs 116. Of course, itis contemplated that fewer or greater numbers of prongs 116 and holes118 may be provided in a wafer 100 within the scope of the presentinvention. For instance, the number of prongs 116 and holes 118 may beadjusted based on the length of the wafer 100.

The superior and inferior endplates 12 and 14 are configured to beinitially releasably engaged when the device 10 is unexpanded, as shownin FIGS. 1 and 2. In this unexpanded condition, the device 10 isattached to a track assembly as described in the '867 patent. In thisstage, the hub 40 is disposed within the cavity 24 of inferior endplate14 with the ribs 34 on the interior surfaces of side walls 16, 18engaging the grooves 58 and 60 extending along each side of the hub 40.The lower surface 50 of hub 40 is on or closely adjacent to the wafersupport ledges 28 in facing relationship. This engagement temporarilyholds the superior and inferior endplates together as the device 10 isintroduced into the intradiscal space to be distracted. In thisunexpanded condition the outer surface 12 a of the superior endplate 12is substantially flush with the upper surfaces 16 a and 18 a of thesidewalls 16 and 18, with the flanges 54 and 56 residing in recesses 20a and 22 a of the rear end wall 20 and the front end wall 22 of theinferior endplate 14, as illustrated in FIGS. 1 and 2. Such nesting ofthe superior endplate 12 within inferior endplate 14 allows for lowerheight of the unexpanded device 10.

The manner in which the interbody fusion device 10 is expanded isillustrated in FIGS. 10-11. When the first wafer 100 is introduced, theinterlocking features on the upper surface 102 of the wafer 100 engagethe mating features 52 on the lower surface 50 of superior endplate 12lifting the superior endplate 12 upwardly within the cavity 24 betweensidewalls 16, 18 and breaking the initial releasable engagement. Whenthe first inserted wafer 100 is introduced into the device 10 the holes118 in the wafer 100 are located to be in alignment and communicationwith the openings 62 b extending through the lower surface 50 ofinferior endplate 12. The locking elements 116 lockingly engage thelower surface 50, one adjacent each of the distal and proximal ends ofsuperior endplate 12 and the generally central locking element lockinglyengaging the lower surface of cross member 64 extending between theopenings 62 b. This process continues with each successive wafer 100inserted beneath a previously inserted wafer 100 until a complete stackis formed, as depicted in FIGS. 10-11. As each subsequent wafer 100 isintroduced, the locking elements 116 lockingly engage the matingfeatures on the lower surfaces of each previously introduced wafer 100,with the holes 118 of each wafer 100 being inserted such that they arein alignment and communication with the holes 118 of each previouslyintroduced wafer 100. The lowermost wafer 100 is supported on thesupport surfaces of ledges 28 with the holes 118 therethrough being inalignment and communication with the openings 36 extending throughinferior endplate 14. It should be noted that preferably all the wafers100, but at least the two lowermost wafers 100, are contained within andconstricted by the opposing side walls 16, 18 and the rear and front endwalls 20, 22 so as to provide additional resistance against torsionalmovement of the spine. The manner in which the expanded interbody fusiondevice 10 is released from the wafer track assembly of the insertioninstrument by the severing of posts 32 is fully described in the '867patent.

Having described the interbody fusion device 10, a suitable bone filleror bone graft to promote fusion between opposing vertebral bodies may beinserted into the expanded device 10 as well as into the intradiscalspace adjacent to device 10. With the instrument used to insert device10 having been removed from the expanded device 10, it can beappreciated that the wafer insertion channel 30 provides access into theexpanded device 10. A suitable graft insertion instrument may be used toinject bone graft under pressure into the expanded device 10. Under anappropriate pressure, such bone graft will flow through the holes 118extending through the wafers 100 and into the openings 36 through theinferior endplate 14 and into the multi-contoured opening 62 through thesuperior endplate 12. The bone graft will also flow into the countersinksurfaces 36 a surrounding the openings 36 so as to further increasecontact area between the bone graft and the endplate of the inferiorvertebral body. Injection of the bone graft will continue until thegraft is stress loaded against the endplates of the opposing vertebralbodies. In some instances, bone graft may be pre-loaded into anunexpanded device 10 prior to insertion of the device 10 into theintradiscal disc space. Suitable bone graft materials may includeautograph bone, allograft bone, bone morphogenic protein (BMP) andxenograft and synthetic derived bone substitutes, as described forexample, in the '998 patent. It should also be understood that amaterial with a bone fusion promoting substance, such as a spongesaturated with BMP, may be placed in the single opening 62 a of themulti-contoured opening 62 and supported by the cross member 64. Thiswill allow the fusion promoting substance to be pre-loaded into device10 and not be disrupted upon expansion of device 10 by insertion ofwafers 100 as described herein.

It is contemplated that each of the components of the device 10, namelythe superior endplate 12, inferior endplate 14 and the wafers 100described herein, be formed of a biocompatible material that issufficiently rigid to form a solid stack as the successive wafers areinserted into the device. Thus, in one specific embodiment, thecomponents are formed of PEEK or a carbon-fiber reinforced PEEK, orsimilar polymeric material. Alternatively, the superior and inferiorplates may be formed of a biological material, such as a bone graftmaterial, or an osteoconductive or osteoinductive material.

In accordance with certain specific applications, the device 10 hasparticular utility as a lateral implant for insertion into theintradiscal space using a lateral approach as more fully described inPCT Application No. PCT/US2012/054055, entitled “Lateral ApproachExpandable Spinal Implant and Method”, filed on Sep. 7, 2012 andcommonly assigned to the same assignee as the present invention, thedisclosure of which is incorporated herein by reference in its entirety.As such, the overall length L of the device 10 as shown in FIGS. 5-6,including the lengths of both the superior endplate 12 and the inferiorendplate 14, is about 37 mm. The width Ws of the superior endplate 12 isapproximately 12.5 mm and the width Wi of the inferior endplate 14 isapproximately 17.3 mm. The height of the unexpanded device 10 of FIGS.1-2 with the superior endplate 12 fully nested within the inferiorendplate 14 is approximately 8 mm. With the introduction of four wafers100, each of which has a thickness of approximately 1.0 mm, the heightof device 10 may be expanded from an unexpanded height of approximately8 mm to an expanded height of approximately 12 mm. Of course, the numberof wafers may vary depending upon the particular surgery and the initialheight may also be different. For example, device 10 may be formed tohave an initial unexpanded height of 9 mm and with the addition of fivewafers 100, each having a thickness of 1 mm, the height of device 10 maybe increased to 14 mm. As such, it should be appreciated that thesedimensions are only illustrative and that the dimensions of the device10 and the number of wafers 100 to be inserted and their thicknesses mayvary depending upon the application.

The footprint of the outer surfaces of the superior and inferiorendplates 12, 14 that contacts the endplates of opposing vertebralbodies is determined by the area defined by the perimeter of such outersurfaces. Thus the footprint of outer surface 12 a is L times Ws, asshown in FIG. 5. In the particular example above with device 10 having a37 mm length, the footprint is approximately 461 mm². The bone graftarea that contacts the endplate of the superior vertebral body isdefined by the area of the single opening 62 a, which in this example isapproximately 95 mm². The ratio of bone graft area to the footprint atouter surface 12 a is therefore about 20.6%. The footprint of the outersurface 14 a of the inferior endplate 14 that contacts the endplate ofthe inferior vertebral body is defined by the area, L times Wi, as shownin FIG. 6. In this particular example, the footprint is approximately639 mm². The bone graft area that contacts the endplate of the superiorvertebral body is defined by the total area of the two openings togetherwith the countersink areas 36 a, which in this example is approximately98 mm². The ratio of bone graft area to the footprint at outer surface14 a is therefore about 15.3%.

Turning now to FIGS. 12-18 a second embodiment of the invention isdescribed. An expandable interbody fusion device 200 comprises asuperior endplate 212, an inferior endplate 214 and a plurality ofinterlocking wafers 300. Components of interbody fusion device 200 aresubstantially the same both structurally and functionally as likecomponents of interbody fusion device 10, except for severaldifferences. The first difference is that device 200 is of smaller sizethan device 10 and is particularly shorter in length. The device 200thus has particular utility as a spinal implant inserted posteriorly orposteriolaterally either bilaterally or unilaterally depending upon thesurgical indication and the surgeon's preference.

The second difference of device 200 over device 10 is that the superiorendplate 212 is not fully nested within the sidewalls 216, 218 and thefront end wall 220 and the rear end wall 222 of device 200. Each sidewall 216, 218 defines upper surfaces 216 a and 218 a extendinglengthwise thereon. Rear end wall 220 defines a recess 220 a extendingtherein at that the upper surface thereof and front end wall 222 definesan upper surface 222 a coplanar with upper surfaces 216 a and 218 a.Superior endplate 212 includes a flange 254 projecting outwardly andlongitudinally from the hub 240 at the rear proximal end surface 246 anda flange 256 projecting outwardly and longitudinally from the hub 240 atthe front distal end surface 248 at the front distal end surface 248.Flanges 257 and 259 project outwardly and laterally from the hub 240from hub side surfaces 242 and 244, respectively. In the unexpandedcondition, the flanges 256, 257 and 259 rest on top of respective uppersurfaces 222 a, 216 a and 218 a with a flange 254 residing in recess 220a. While not fully nested in a manner as provided with device 10, theadded expanse of the flanges 256, 257 and 259 provides for a largerfootprint than the fully nested structure.

A third difference of device 200 over device 10 is that device 200 hasmulti-contoured openings at both the upper surface 212 a and 214 a, asshown in FIGS. 15 and 16. The superior endplate 212 has an opening 262therethrough, the upper portion of opening 262 being a single,oval-shaped opening 262 a. The lower portion of opening 262 comprises apair of generally rectangular openings 262 b separated by a cross member264 disposed generally centrally and laterally across superior endplate212. Openings 262 b extend generally linearly along the longitudinaldirection of superior endplate 12. The inferior endplate 214 has anopening 236 therethrough, the lower portion of opening 236 being asingle, oval-shaped opening 236 a. The upper portion of opening 236comprises a pair of generally rectangular openings 236 b separated by across member 238 disposed generally centrally and laterally acrossinferior endplate 214. Openings 236 b extend generally linearly alongthe longitudinal direction of superior endplate 12 and are disposedtherethrough such that in assembly with superior endplate 212 openings236 b are generally in alignment in the elongate direction with openings262 b through superior endplate 212. It should be appreciated that thearea of bone graft contact with the endplates of opposing vertebralbodies may be maximized by the use of the multi-contoured openings 236and 262 each of which has a substantially large single oval-shapedopening 236 a and 262 a, respectively, while maintaining strength of theendplates 212, 214 due to the cross members 238 and 264.

A fourth difference of device 200 over device 10 is that the ribs 212 band 214 b are configured to include a saw tooth shape rather than thepyramidal configuration of the ribs 12 b and 14 b of the device 10.

The expansion members defined by interlocking wafers 300 aresubstantially similar to wafers 100 except for size and are inserted ina similar manner such that once inserted the holes 318 are aligned andin communication with openings 236 b through the inferior endplate 214and with openings 262 b through the superior endplate 212.

In an example of the second embodiment, the overall length L of thedevice 200 is about 25 mm and the overall width is approximately 12 mm.The height of the unexpanded device 10 of FIGS. 12-13 is approximately 7mm. With the introduction of each wafer 300 having a thickness ofapproximately 1.0 mm, the height of device 200 may be expanded in 1 mmincrements. As such, the insertion of one wafer 300 would increase theheight of device 200 to 8 mm while the addition of three wafers 300would increase the height to 10 mm. As with device 10, it is preferablethat all the wafers 300, but at least the two lowermost wafers 300 wheremore than a single wafer is inserted, be contained within andconstricted by the opposing side walls 216, 218 and the rear and frontend walls 220, 222 so as to provide additional resistance againsttorsional movement of the spine. It should be appreciated that with thisembodiment other unexpanded starting heights and lengths of device 200may be contemplated as well as different number of wafers 300 and waferthicknesses, depending upon the particular application. For example, adevice having an overall length of approximately 29 mm and a width ofapproximately 12 mm may have an unexpanded height of about 9 mm withthree wafers 300 inserted to thereby increase the height to about 12 mm.

In the particular example above with device 200 having a 25 mm length,the footprint of outer surface 212 a of superior endplate 212 isapproximately 283 mm². The bone graft area that contacts the endplate ofthe superior vertebral body is defined by the area of the single opening262 a, which in this example is approximately 78 mm². The ratio of bonegraft area to the footprint at outer surface 212 a is therefore about27.6%. The footprint of the outer surface 214 a of the inferior endplate214 that contacts the endplate of the inferior vertebral body in thisparticular example is approximately 305 mm². The bone graft area thatcontacts the endplate of the inferior vertebral body is defined by thearea of the single opening 236 a, which in this example is approximately78 mm². The ratio of bone graft area to the footprint at outer surface214 a is therefore about 25.6%. Accordingly, the ratio of bone graftarea to the footprint at the outer surfaces of the expandable interbodyfusion devices in the examples ranges from about 15 to 28%.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same should be considered asillustrative and not restrictive in character. It is understood thatonly the preferred embodiments have been presented and that all changes,modifications and further applications that come within the spirit ofthe invention are desired to be protected. For instance, the superiorand inferior endplates of the expandable interbody fusion device mayeach have a single opening extending therethrough in communication andalignment with at least one expansion members defined by an interlockingwafer. Such wafer would have at least two locking elements thereon, onelocking element being located on each side of the hole through suchwafer, such that a locking engagement would be provided at each of theproximal and distal ends of the device. Also, while the illustratedembodiments have been directed to interbody fusion of the spine, theexpandable devices and wafers disclosed herein may be used in otherapplications that require distraction of tissue surfaces. Modificationsin size may be necessary depending upon the body space being distracted.

What is claimed is:
 1. In an expandable interbody fusion device forimplantation into the intradiscal space between two opposing vertebralbodies of a spine of the type comprising a first endplate having anouter surface defining a first perimeter configured to contact one ofsaid vertebral bodies, a second endplate having an outer surfacedefining a second perimeter configured to contact the other of saidvertebral bodies, and an expandable structure therebetween to separatesaid first endplate and said second endplate and effect expansion ofsaid device, the improvement wherein: said first endplate has amulti-contoured opening extending therethough and fully disposed withina respective said first perimeter, said multi-contoured opening having asingle opening through the outer surface of said first endplate and atleast two openings through a lower surface of said first endplate andcommunicating with said single opening, said two openings at the lowersurface of said first endplate being spaced by a cross member extendingtransversely across the first endplate at the lower surface thereof,said cross member defining at the lower surface thereof a shapedconfiguration with mating features for interlocking engagement with saidexpandable structure.
 2. The improvement of claim 1, wherein said secondendplate has an upper support surface generally opposite said secondendplate outer surface, said second endplate and said first endplatebeing initially disposed in a first position such that said lowersurface of said first endplate and said upper support surface of saidsecond endplate generally face each other, said second endplate havingat least one opening extending through said outer surface and throughsaid upper support surface.
 3. The improvement of claim 2, wherein saidexpandable structure comprises at least one expansion member sized to bereceived into said device and supported on said upper support surfacebetween said first endplate and said second endplate, and upon receipttherein to move said first endplate and said second endplate relativelyapart.
 4. The improvement of claim 3, wherein said expansion member hasa hole extending therethrough and includes a mating feature thereonalong the direction of receipt, said expansion member forming uponreceipt into said device an interlocking interface between said matingfeature of said expansion member and the shaped configuration of saidcross member of said first endplate, said hole being located such thatupon receipt of said elongate expansion member into said device saidhole communicates with said multi-contoured opening through said firstendplate and said at least one opening through said second endplate. 5.The improvement of claim 4, wherein said first endplate is elongate andsaid two openings at said lower surface extend along the elongatedirection and are spaced by said cross member which extends transverselyacross the first endplate at the lower surface thereof.
 6. Theimprovement of claim 5, wherein said expansion member has three lockingelements and two holes located generally linearly along the elongatedirection, one locking element being disposed adjacent each end of theexpansion member and one locking element being disposed generallycentrally with one hole being disposed on each side of the generallycentrally disposed locking element, said two holes being located suchthat upon receipt of said elongate expansion member into said devicesaid holes communicate with said two openings at the lower surface ofsaid first endplate.
 7. The improvement of claim 6, wherein saidgenerally centrally located locking element lockingly engages the shapedconfiguration of the lower surface of said first endplate at said crossmember.
 8. The improvement of claim 7, wherein said second endplate iselongate and has at least two openings extending through said outersurface and said upper support surface extending along the elongatedirection, said two openings being located such that upon receipt ofsaid elongate expansion member into said device said two openingscommunicate with said two holes through said expansion member.
 9. Theimprovement of claim 8, wherein each of said locking elements comprisesa resiliently deflectable locking surface.
 10. The improvement of claim2, wherein said second endplate has a pair of opposing spaced apartsidewalls projecting upwardly from said upper support surface towardsaid first endplate.
 11. The improvement of claim 10, wherein said firstposition of said first endplate and said second endplate is defined by areleasable engagement between said first endplate and said sidewalls.12. The improvement of claim 10, wherein said expansion member isdefined by a generally elongate flat wafer having an upper surface andan opposing lower surface with side edges therebetween.
 13. Theimprovement of claim 12, wherein said device comprises at least onefurther wafer received between said wafer and said upper support surfaceof said second endplate, said further wafer defining a resilientinterlocking interface with said wafer, said wafer and said furtherwafer being contained within the sidewalls of said second endplate. 14.The improvement of claim 2, wherein said at least one opening extendingthrough said outer surface and said upper support surface of said secondendplate is a multi-contoured opening.
 15. The improvement of claim 14,wherein said multi-contoured opening through said second endplate has asingle opening through the outer surface of said second endplate and atleast two openings through the upper support surface of said secondendplate communicating with said single opening.
 16. The improvement ofclaim 15, wherein said second endplate is elongate and said two openingsat said upper support surface extend along the elongate direction. 17.The improvement of claim 16, wherein said two openings at the uppersupport surface of said second endplate are spaced by a cross memberextending transversely across the second endplate at the upper supportsurface thereof.
 18. The improvement of claim 17, wherein the perimeterof the outer surfaces of each of said first endplate and said secondendplate defines a respective footprint, and wherein each of the atleast one opening of said first endplate and said second endplatedefines a bone graft area for contacting opposing vertebral bodies. 19.The improvement of claim 18, wherein the ratio of the bone graft area tothe footprint of at least one of said first endplate and said secondendplate ranges from about 15 to 28%.